The Branching and Innervation Pattern of the Radial Nerve in the Forearm: A Study to Clarify the Literature

Recent experience in a medical gross anatomy lab has indicated that different opinions exist among common references regarding which branches of the radial nerve innervate which muscles, and in what order. This study attempts to clarify the innervation pattern(s) of the muscles of the posterior forearm through a survey of the existing standpoints on the subject and in‐person cadaveric dissection. METHODS Anatomical textbooks, atlases, and peer‐reviewed literature detailing the complete course of the radial nerve in the forearm were surveyed to identify their positions on the muscular territory of each branch of the radial nerve and the order of innervation of the muscles of the posterior forearm. Twenty cadavers, with a goal of thirty‐five, were dissected to expose the radial nerve in the forearm and the spiral groove of the arm. A pin was placed through the radial nerve at the point where it crossed a line bisecting the medial and lateral humeral epicondyles (the transepicondylar line, TEL). Each muscular branch of the nerve was identified, and its length and the distance of its branch point from the TEL were recorded to the nearest one‐hundredth of a millimeter using a digital caliper (Neiko Tools USA). RESULTS Review of the literature (5 texts, 4 articles) yielded 3 different primary branching orders and 5 different primary innervation patterns. Conflict was most notable in regards to the innervations of extensor carpi radialis brevis and supinator, and their order relative to each other. Additionally, one textbook did not differentiate the territories of the deep branch of the radial n. from the posterior interosseus n., and all of the peer‐reviewed articles referred to both nerves as simply the posterior interosseous n. Preliminary cadaveric dissection (N=20) yielded varying branching orders and muscle innervations. The most consistent branch order was brachialis, brachioradialis, extensor carpi radialis longus (ECLR), extensor carpi radialis brevis (ECRB), supinator, extensor digitorum (ED), extensor carpi ulnaris (ECU), extensor digiti minimi (EDM), abductor pollicis longus (APL), extensor pollicis brevis (EPB), extensor pollicis longus (EPL), and extensor indicis (EI). The most common sources of innervation were as follows: radial nerve proper supplied innervation to brachialis (65%), brachioradialis (100%), ECRL (100%), and ECRB (50%). Deep branch of the radial n. supplied ECRB (40%) and supinator (90%). Superficial branch of the radial n. supplied ECRB in 25%. Posterior interosseous n. supplied ED (95%), ECU (85%), EDM (95%), APL (95%), EPB (100%), EPL (100%), and EI (100%). CONCLUSION The radial nerve exhibits significant variability in the posterior forearm. However, there was enough consistency across the specimens studied to identify an archetypal pattern and order of innervation. Taking this into consideration, it could be argued that there is an unwarranted level of disagreement between the major resources commonly used by students studying clinical anatomy, which could potentially lead to undue confusion. These findings may also need to be considered when planning surgical approaches to the distal arm, elbow and proximal forearm to avoid undue loss of motor function.